The invention relates to the production of metallic flake aluminum pigments.
Aluminum flake pigments are used as silver metallic pigments in paints and ink industries. The flake pigments are referred to as leafing and non-leafing types. They may be in the form of dry powders or pastes. Aluminum pigments are conventionally manufactured by milling aluminum of various available physical forms, for example, granular or spherical particles, foils or shredded foils, in hydrocarbon liquids. Fatty acids are used as lubricants in the process. With the addition of a small quantity of a suitable lubricant in a milling or grinding medium consisting of an inert hydrocarbon liquid, which is conventionally known as mineral spirit, slurries comprising flaked aluminum pigments of desired product quality are produced. Depending upon the nature of milling actions for example the time, speed, temperature, milling media, feed rate and charge weight ratios, the final configuration of the flaked pigment particles and the degree of particle fineness are determined. However, due to the randomness of the process the aluminum flake slurries thus produced contain a very wide distribution of flake particle sizes. Thus, in order to produce flake pigments of desired size distribution, it is required to screen the slurries to remove the coarser sized particles. The screening process is one of the major process steps that control the quality of a particular pigment type. After screening, the slurry is further concentrated as filter cakes by removing most of the hydrocarbon liquid through a filter press. The filter cakes are then blended as pastes of desired aluminum metal pigment concentration.
One problem with these conventional methods of manufacture is that hydrocarbon solvents, for example, mineral spirits containing volatile organic, both small and medium range aliphatic and complex aromatic molecules are used. Because of the nature of the process there is always some solvent discharge to the atmosphere. There is growing concern over discharging organics into the environment. They can form various toxic radicals through complex photochemical processes.
Conventionally, the slurries are screened to remove coarser sized particles using vibratory screens, for example those available from Sweco. These are generally open systems in which the product slurries are discharged on a wire mesh of certain permissivity mounted on a vibratory bed. The screened slurries are collected for product processing and the screen residue, hereinafter referred to as oversize, is collected separately for reprocessing. To increase the screening efficiency and to avoid screen blocking, the product slurries are often diluted by adding more solvents while screening. The major drawbacks of this system include:
a) high over-size build-up on the screen,
b) requires large space,
c) scope of solvent emission is always higher (can be controlled but cannot be eliminated),
d) limited production yield of finer grade products by multi-stage screening, and
e) longer processing time.
Generally, the oversize produced by such conventional screening systems contains a large portion of entrapped products that fail to pass through the screen because of the increased restrictions of the available screen surface area as screening progresses. For fiber recovery of the product from the oversize, the oversize is diluted to a desired strength and re-screened. This is highly inefficient.
There is therefore a need for an improved method for the production of flake aluminum pigments which will address these problems.
According to the invention there is provided a method of manufacturing metallic flake pigments with improved product properties comprising:
comminuting metallic flake particles in an inert solvent; and
in-line screening of the slurry thus formed on an in-line screening device.
The method of the invention provides considerable operational advantages because of its flexibility, efficiency, safety and environmental friendliness. The finished product produced by the method of the invention has consistent flake particle size distribution with brighter metallic finishes than products produced using conventional techniques. The method allows several different products with different flake particle size and optical properties to be produced. The method also allows selective production of a flake pigment by in-line particle size classification(s). Productivity and product quality are greatly improved.
In a particularly preferred embodiment of the invention the in-line screening is carried out by passing the slurry through a cylindrical screen.
Preferably the method includes the step of rotating the screen.
In one embodiment of the invention the in-line screening comprises at least two sequential in-line screening devices with screens of different mesh size. Preferably, the mesh size of the screens decreases in the downstream direction.
In a preferred embodiment of the invention the method includes the steps of:
periodically closing the flow of slurry to the in-line screening device;
flushing the screening device in-line; and
stopping flushing and continuing the flow of slurry to the flushed in-line screening device.
In this case preferably the method includes the step of monitoring the pressure drop across the screening device, stopping the flow of slurry and commencing in-line flushing in response to a pressure drop above a pre-set value.
Preferably the method includes the step of recycling oversize material from the in-line screening device.
The metallic flake pigments are usually aluminum flake pigments which may be of a leafing or non-leafing category.
In one embodiment of the invention the method comprises the step of mixing the residues from at least some of the screening units.
The invention also provides metallic flake pigments produced by the method of the invention.